Recently, a high exhaust gas purifying performance is requested in an internal combustion engine. Especially in a diesel engine, it is important to eliminate particulate matters (PM) emitted from the diesel engine. Usually, a diesel particulate filter (DPF) is provided in an exhaust pipe to eliminate the PM.
In order to improve the exhaust gas purifying performance, a configuration in which a PM sensor is provided downstream of the DPF may be necessary. By detecting an output value of the PM sensor, it can be determined whether a PM amount in the exhaust gas downstream of the DPF is decreased enough to a low level or whether the DPF is abnormal.
It is necessary that the PM sensor itself is normal so as to properly perform the above processing. Therefore, it is also important to properly detect whether the PM sensor is abnormal. JP-2010-275977A discloses an abnormality detector of the PM sensor.
The abnormality detector performs the abnormality diagnosis of the PM sensor in a time period just after a collecting rate of the DPF is low after the DPF is regenerated. It is not preferable that a time period for performing the abnormality diagnosis is limited.
Further, because the DPF is provided upstream of the PM sensor, it is difficult for the abnormality detector to determine whether the PM sensor is abnormal or whether the collecting rate is high so that the output value of the PM sensor is low. In a case where the collecting rate is high just after the DPF is regenerated, the abnormality detector may erroneously determine that the PM sensor is abnormal. Therefore, it is preferable that the abnormality diagnosis can be accurately performed without being affected by the collecting rate.